The Spanish Civil War (1936–1939) broke out with a military uprising in Morocco on July 17, triggered by events in Madrid. Within days, Spain was divided in two: a "Republican" or "Loyalist" Spain consisting of the Second Spanish Republic (within which were pockets of revolutionary anarchism and Trotskyism), and a "Nationalist" Spain under the insurgent generals, and, eventually, under the leadership of General Francisco Franco.
By the summer, important tendencies of the war become clear, both in terms of atrocities on both sides and in the contrast between the Soviet Union's intermittent help to the Republican government and the committed support of Fascist Italy and Nazi Germany for the Nationalists.
In the early days of the war, over 50,000 people who were caught on the "wrong" side of the lines were assassinated or summarily executed. In these paseos ("promenades"), as the executions were called, the victims were taken from their refuges or jails by armed people to be shot outside of town. Probably the most famous such victim was the poet and dramatist Federico García Lorca. The outbreak of the war provided an excuse for settling accounts and resolving long-standing feuds. Thus, this practice became widespread during the war in areas conquered. In most areas, even within a single given village, both sides committed assassinations.
Any hope of a quick ending to the war was dashed on July 21, the fifth day of the rebellion, when the Nationalists captured the main Spanish naval base at Ferrol in northwestern Spain. This encouraged the fascist and other sympathetic regimes of Europe to help Franco, who had already contacted the governments of Germany and Italy the day before. On July 26, the future Axis Powers cast their lot with the Nationalists. German assistance, channeled through Franco rather than to the Nationalists in general, consolidated his leadership position of the insurgency.
In the north, a rebel force under Colonel Beorlegui, sent by General Emilio Mola, advanced on Gipuzkoa. On September 5, after heavy fighting, it took Irún, closing the French border to the Republicans. On September 13 the Basques surrendered San Sebastián to the Nationalists, who then advanced toward their capital, Bilbao, but were stopped at the border of the province. The capture of Gipuzkoa had isolated the Republican provinces in the north.
Nationalist forces under Franco won a great symbolic victory on September 27 when they relieved the besieged Alcázar at Toledo. Two days after relieving the siege, Franco proclaimed himself Generalísimo and Caudillo ("chieftain"); he would forcibly unify the various Falangist and Royalist elements of the Nationalist cause.
In October, the Nationalists launched a major offensive toward Madrid, reaching it in early November and launching a major assault on the city on November 8. The Republican government was forced to shift from Madrid to Valencia, out of the combat zone, on November 6. However, the Nationalists' attack on the capital was repulsed in fierce fighting between November 8 and 23. A contributory factor in the successful Republican defence was the arrival of the International Brigades, though only around 3,000 of them participated in the battle. Having failed to take the capital, Franco bombarded it from the air and, in the following two years, mounted several offensives to try to encircle Madrid.
On November 18, Germany and Italy officially recognized the Franco regime, and on December 23, Italy sent "volunteers" of its own to fight for the Nationalists.